Passing a parameter using RelayCommand defined in the ViewModel (from Josh Smith example)

Question

I would like to pass a parameter defined in the XAML (View) of my application to the ViewModel class by using the RelayCommand. I followed Josh Smith\'s excellent article o

Answer 1

Nothing posted here before worked for me.

Turns out, all answers are missing the <object> after RelayCommand!

This works for me:

public RelayCommand<object> OKCommand
{
    get
    {
        if (_okCommand == null)
            _okCommand = new RelayCommand<object>(OkCommand_Execute);
        return _okCommand;
    }
}
private RelayCommand<object> _okCommand = null;

private void OkCommand_Execute(object obj)
{
    Result = true;
}

If you want to use aCanExecute method, use the following code:

_okCommand = new RelayCommand<object>(OkCommand_Execute, OkCommand_CanExecute);

private bool OkCommand_CanExecute(object obj) { }

Answer 2

You'll pass the param in the lambda to the command like so:

if (_aCommandWithAParameter == null)
{           
    _aCommandWithAParameter = new RelayCommand(               
        param => this.CommandWithAParameter(param)
        );        
}

Answer 3

I don't understand why you have the extra complexity of specifying the lambda in the first place. Why not just do this:

if (_aCommandWithAParameter == null)
{           
    _aCommandWithAParameter = new RelayCommand<object>(CommandWithAParameter);
}

private void CommandWithAParameter(object state)
{
    var str = state as string;
}

Answer 4

I am just trying to sell my point, check this out whether this works...

http://mywpf-visu.blogspot.com/2009/12/relay-command-how-to-pass-parameter.html

Answer 5

Here is a simple solution for the commandparameter as I was looking for help on the subject. I could not find anything online that was simple enough. The following solution works well when you are using a relaycommand. I had a few hyperlinks for which I needed to get the url value that was clicked using the command parameter.

Step 1: In your relay command, create a simple property that will hold the parameter object value. You could call it parametervalue or any name that you prefer.

public object ParameterValue
{
  get;
  set;
}

Step 2: In the Execute Method of the RelayCommand class, set the value or the property created above to the parameter from the Execute method.

readonly Action<object> m_execute;       // Action to execute

public void Execute(object parameter)
 {
   this.ParameterValue = parameter;
   m_execute(parameter);
 }

Step 3: Now when you can bind the CommandParameter in xaml to any value you want to retrieve when the command is executed. example:

<TextBlock>
  <Hyperlink Command="{Binding Path=NavigateUrlCmd}"
             CommandParameter="{Binding ElementName=tbwebsite, Path=Text}">
    <TextBlock Name="tbwebsite" Text="{Binding Path=website}"/>
  </Hyperlink>
</TextBlock> 

If you have a command called chickenCommand, when executed you could access the parameter: chickenCommand.ParameterValue

I hope this helps somebody. Thank you for all your previous help.

Answer 6

I cannot substitute a reference to the method name for the lamda expression withing a compile error. Apparently, and by no means surprisingly, a non-static method name reference cannot be used in place of a lambda. I hardly see it as "added complexity". Consistently passing lamdas makes sense to me.